This invention relates to a spray device used for a reactor core of a nuclear reactor to spray cooling water to the upper portion of the reactor core for rapidly cooling it in the event of loss of coolant accident (LOCA).
In a nuclear reactor, particularly a light water type nuclear power reactor, in which a reactor core is contained in a reactor pressure vessel and surrounded by a shroud, LOCA may occur because of breaking of a water recycle duct system resulting in flow out of water from the reactor core. At this time, water level lowers to such an extent as fuel assemblies are exposed and the reactor core runs in a condition of less or no coolant (hereafter called coolant-less operation), whereby the fuel assemblies are overheated and damaged.
In order to prevent the coolant-less operation of the reactor core, a spray device is usually arranged above the upper portion of the reactor core to supply the cooling water. In a spray device of the usual horizontal type, it is arranged along the peripheral wall of the shroud and between the upper portion of the reactor core and a shroud head detachably mounted on the upper portion of the shroud to close it.
In the spray device of this horizontal type, cooling water is sprayed horizontally towards the central axial portion of the reactor core through nozzles attached to a circular duct, i.e. sparger, located along the periphery of the shroud.
However, the spray device of the horizontal type involves serious and difficult problems for uniformly spraying cooling water to reach the central axial portion of the reactor core in the event of LOCA. Namely, steam blown up from the reactor core is condensed by cooling water horizontally sprayed and the cooling water therefore hardly reaches the central axis portion of the reactor core so that the cooling water cannot be uniformly distributed throughout the entire portion of the reactor core. Moreover, temperature distributions in the reactor core differ along the radial direction at the peripheral portion and the central portion, so that there is a problem that sufficient quantity of the cooling water cannot be supplied to the central portion of the reactor core which requires most efficient cooling for the reason that the fuel assemblies located near the periphery of the reactor core is run at a relatively low power and those near the central portion thereof is run at a relatively high power. Therefore, the sprayed water easily falls down at the periphery of the reactor core, and in addition, the blown-up steam is largely condensed by the sprayed cooling water near the periphery of the reactor core and hardly condensed at the central portion thereof because subcooled temperature is high near the periphery and nearly zero at the central portion. Consequently, the cooling efficiency at the central portion is extremely lowered.
In order to obviate defects described above, it has been proposed to construct and use a spray device so as to eject a large amount of cooling water under pressure so that sufficient amount of cooling water can reach the central portion of the reactor core, but the spray device of this type involves problems regarding arrangement and distribution of spray nozzles as well as design and size of the spray device necessary to supply the cooling water effectively and uniformly.
In order to obviate problems encountered by the conventional spray device of the horizontal type described above, spray devices of vertical type was proposed in which cooling water is sprayed from a portion above the reactor core by taking into consideration the temperature distribution and sprayed amount of the cooling water along the radial direction of the reactor core.
One conventional spray device of the vertical type is attached to the shroud head so as to spray cooling water downwardly onto the reactor core and this spray device is connected to a duct attached to and extending through the wall of the reactor pressure vessel for supplying cooling water to the spray device. However, with the spray device of this type, it is necessary to change the construction of the shroud head, and moreover, at the time of mounting or dismounting the shroud head, it is troublesome to connect or disconnect the spray device to or from the duct for supplying cooling water to the spray device.
The same problems as described above are encountered in another spray device of the vertical type in which the spray device provided with nozzles is stretched on the inner side of the shroud head to downwardly spray cooling water. In addition, the weight of the shroud head increases.